There is a case where a first resin member (e.g., a rectangular parallelepiped box-shaped casing main body which is formed of a thermoplastic resin such as nylon, which has a rectangular shape in plan view, and which is open above) is intended to be sealed with a second resin member (i.e., a cover (lid) which is formed of, for example, nylon and which has a rectangular flat portion having a uniform thickness) so as to close the opening of the first resin member, by means of laser welding (hereinafter may be referred to simply as “welding”) of the peripheral surface (on the opening side) of the casing main body (i.e., the upper end surface of the side wall (vertical wall) along the opening) and the peripheral edge surface (the peripheral-end-side surface) of the cover which abuts the peripheral surface of the casing main body. The aforementioned case corresponds to, for example, a case where the thus-produced casing is employed for a controller including therein a control circuit board for an in-vehicle electric component (e.g., a glow plug or a gas sensor). In such a case, while an appropriate surface pressure is applied, by means of a jig or a similar apparatus, to the contact interface between the resin members to be welded, a laser (e.g., a semiconductor laser) head is positioned at a specific position (start point) of a portion to be welded (hereinafter such a portion may be referred to as a “welding portion”) on the outside of the cover (on the side opposite the bonding surface). Then, while the cover is irradiated with laser light, the laser head is circumferentially moved along the opening, to thereby carry out laser welding along a specific line (Patent Document 1). In the case of such laser welding, the laser light transmitting through the second resin member heats and melts a bonding interface portion of the first resin member (absorbable thermoplastic resin), the portion being in contact with the second resin member (transmissible thermoplastic resin), and the thus-generated heat melts the corresponding bonding interface portion of the second resin member, whereby the bonding interface portions are welded; i.e., the circumferential portions of the resin members are welded in association with movement of the laser head.
For reliable sealing of the opening of the casing main body through such laser welding, the end point of movement (circumferential movement along the (rectangular) opening) of the laser head (i.e., end point of laser irradiation (welding end position)) must fall beyond the start point thereof (i.e., start point of laser irradiation (welding start position)) by an appropriate length (e.g., 2 to 3 mm), in order to prevent occurrence of a very small unwelded portion, which would otherwise occur between the start point and the end point when the end point does not fall beyond the start point. Thus, when laser welding is carried out as described above, a portion between the start point and the end point (having a specific length (e.g., 2 to 3 mm)) is subjected to the laser irradiation process twice; i.e., the portion is subjected to the welding process twice (hereinafter the portion may be referred to as a “weld overlap portion”).
In the welded portions of the resin members—which are bonded together through heating and melting in association with movement of the laser head in the aforementioned welding process—expansion occurs due to heating, and then shrinkage (sink) occurs in a cooling/solidification process. When each of the resin members to be welded is an integrally molded resin product, welding portions of the resin member basically exhibit uniform laser light transmissibility or absorbability, or there is virtually no difference in laser light transmissibility or absorbability between the welding portions. Therefore, in the aforementioned welding process, basically, while laser light is radiated at constant laser output, the laser head is moved (scanned) at constant speed, in order to secure uniform bonding strength (fixing strength) or sealing performance in the entire welding portions by means of uniform welding.